In Duskwood, directly behind the tower where you can obtain the Apothecary quest that takes you into Stonard, (from two Undead there) there's a cute little place with an archway. (Roughly 86,40 coords). I used to go there to RP all the time. It's also an excellent cut through for Horde to get around Darkshire, from the road through Deadwind Pass.

You'll recognize it because there's a small little glow to the area even at night, like some 'hallowed place' or whatnot. There are no mobs in it at all. It looks like it has a 'garden' area, with lattice work.

T urning left, there's a waterfall (to the left of the archway after you pass through it). If you go to the furthest right rock of that waterfall (going around the fallen trunk on the left side moving right), on your mount, jump and fall off towards the water (not the log at the bottom), you should get the achievement.